The present invention relates to a control apparatus of an AC/DC power converter, which is responsive to occurrence of a single-phase open fault in the power line of an AC system connected to the AC/DC power converter.
FIG. 1 schematically shows a typical AC/DC power transmission system. AC power lines 1 and 1' are coupled via transformers 2 and 2' to converters 3 and 3', respectively. Each of converters 3 and 3' may be formed of a plurality of thyristors connected to a series-parallel fashion. Power conversion from AC to DC or vice versa is carried out by controlling the turn-on phase of each thyristor. In the system of FIG. 1, smoothing reactors 4 and 4', DC power transmission line 5, potential transformers 6 and 6' and current transformers 7 and 7' are further provided. This system also includes a control unit which is formed of automatic current regulators 8 and 8' and automatic margin angle controllers 9 and 9'. This control unit may further include automatic voltage regulators for keeping the DC voltage at a constant value.
The signal values obtained from automatic current regulator 8 or 8' and from automatic margin angle controller 9 or 9' are fed as control voltages to minimim value selector 10 or 10'. Minimum value selector 10 or 10' automatically selects one of the control voltages which serves to advance the control angle to the greatest extent. The selected control voltage from minimum value selector 10 or 10' is fed to control voltage limiter 11 or 11'. This limiter is provided for limiting the peak potential of a control signal input to phase control circuit 12 or 12'. Phase control circuit 12 or 12' supplies turn-on pulses to converter 3 or 3', so that each turn-on phase of the thyristors is determined in accordance with the control signal input to circuit 12 or 12'.
In the above AC/DC power converter configuration, as is well known in the art, one (3) of the two converters is operated as a rectifier with automatic current regulation while the other (3') is operated as an inverter with automatic margin angle control, according to the proper change of current margin .DELTA.I.
In the following description, for the sake of simplicity, converters 3 and 3' are referred to as rectifier and inverter, respectively.
It is now assumed that a single-phase ground fault has occured in a power line of the AC system connected to AC power line 1'. At this time, an AC power line protection unit (not shown) functions to open breakers (not shown) connected to the faulted power line (1'). Thus, a single-phase line open occurs at the faulted power line. Then, the protection unit closes the breaker after the elapse of a predetermined period of time, so called a deionization interval. This is known as single-phase reclosing.
If the operation of the AC/DC converter is continued in the single-phase line-open condition, a large overvoltage occurs at the opened power line (1'). This overvoltage is liable to damage converters 3 and 3' and/or AC filters (not shown). The influence of such overvoltage is particularly pronounced when a weak AC power system is connected to the AC/DC power converter. This is chiefly because an AC impedance seen from the AC/DC converter side is greatly increased when the single-phase line open occurs.
When the single-phase line open occurs on the side of inverter 3' and particularly when a large prefault transmission power is high, a commutation failure of the thyristors is caused. Such a commutation failure serves as an additional disturbance that adversely affects the AC power system, resulting in the occurrence of AC and DC interaction. In short, this disturbance repeatedly causes continuous commutation failures.
When the single-phase line open occurs on the side of rectifier 3 and particularly when the prefault transmission power is low, the DC current flowing through DC line 5 is intermitted. This current intermittence also serves as an additional disturbance that adversely affects the AC power system, resulting in the occurrence of AC and DC interaction. Such a disturbance will induce unfavorable subsequent DC current intermittence. During the transient period of the current intermittence, there is a certain tendency for the automatic current regulation and automatic margin angle control to interfere with each other to promote so called "hunting".
The unfavorable phonomena noted above are common in simulation tests. In addition, overvoltage generation is described in a recently announced treatise (Cigre, International Conference on Large High Voltage Electronic Systems, 1984 Session).